For many diagnostic applications colour or colour change is used as an indication of presence or absence of an analyte in a biologic test sample, such as blood (serum, plasma, whole blood), saliva, tear fluid, urine, cerebrospinal fluid, sweat, etc.
For point-of-care applications where it is desirable to obtain a quick read-out of a test, it is essential that the read-out is clear and unambiguous. There are many applications commercially available which are based on simple test strips onto which a sample is applied, whereupon migration of the sample in the strip brings analytes in contact with a reagent, the response being a colour in case of a positive reaction, i.e. presence of the analyte in question.
It goes without saying that the more reliable, i.e. clear and unambiguous the response is the better it is. Sometimes a test is of a binary nature, such as for pregnancy tests, where there can be only a positive or a negative result. On the other hand, for tests such as allergy tests, there can be a range of levels of the response in dependence on how high the concentration of antibodies in the patient's serum is.
A common marker for use in e.g. allergy or autoimmune tests is a so called gold conjugate, comprising a colloidal gold particle in the nanometer size range, to which a protein (antibody or antigen) has been coupled (“conjugated”). The conjugate gives rise to some detectable signal when the desired analyte is coupled thereto in the assay.
For certain samples where a clear colour indication normally is obtained if there is a positive response, it can sometimes happen that there are other species (frequently the allergens themselves) present that will give rise to gray lines which can be difficult to distinguish from pink lines. This “blurs” the true signal. Thus, one can obtain false positive or false negative results, which of course can be of consequences if the test is to form the basis for a treatment regimen.